Borewit / music-metadata / 31519450225

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fix(mp4): locate each sample entry after the first correctly

A SampleEntry extends Box, so its size field covers itself
(ISO/IEC 14496-12, 8.5.2). StsdAtom advanced the cursor by the full size
after having already stepped over the 4-byte size field, overshooting by
4 bytes for every entry.

The first entry is unaffected, so this only shows up once a sample
description holds more than one entry, which no existing test sample
does. From the second entry onwards the data format is read from the
reserved bytes, giving an all-zero FourCC:

  FieldDecodingError: FourCC contains invalid characters: 00 00 00 00

#2340 corrected the length passed to SampleDescriptionTable on the line
above, but left the cursor advance inconsistent with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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